1. Field of the Invention
The invention relates to a process for the preparation of alkali metal salts of ether-carboxylic acids of the general formula I EQU R--(OC.sub.m H.sub.2m).sub.n --O--CH.sub.2 COOM (I)
in which
R denotes an alkyl group having 1 to 22 carbon atoms, an aryl group or an aralkyl group, PA1 m denotes the number 2 and/or 3, PA1 n denotes a number in the range from 0 to 20 and PA1 M denotes an alkali metal from the group formed by lithium, sodium and potassium, PA1 R, m and n are as defined above,
by oxidation of ether-alcohols of the general formula II EQU R--(OC.sub.m H.sub.2m).sub.n --OCH.sub.2 CH.sub.2 OH (II)
in which
in the aqueous phase with oxygen or gases containing oxygen at elevated temperatures in the presence of alkali metal hydroxides and noble metal catalysts.
2. Statement of Related Art
Alkali metal salts of ether-carboxylic acids are compounds which have intersecting surface-active properties and are employed in the form of their aqueous solutions, for example in cosmetics formulations.
It is known that alkali metal salts of ether-carboxylic acids of the general formula I can be prepared by catalytic oxidation of the corresponding ether-alcohols of the general formula II, compare EP-B-0,039,111, EP-B-0,018,681, EP-B-0,073,545, US-C-3,342,858, DE-C-2,816,127, DE-A-3,135,946, DE-A-2,936,123 and DE-A-3,446,561.
However, only dilute solutions of the alkali metal salts of the ether-carboxylic acids can be prepared by the known catalytic processes. In particular, if oxygen or a gas containing oxygen is passed into a relatively highly concentrated solution of the ether-alcohols in water in the presence of the catalysts, the viscosity of the reaction mixture increases greatly as the conversion increases, passes through a maximum at about 30% conversion (about 30% of sodium salt of the ether-carboxylic acid and about 70% of ether-alcohol) and then drops greatly again as the conversion becomes higher; compare DE-C-2,816,127. The rate of reaction becomes so slow during this procedure, because the mass transfer is impeded by the viscosity, that the process is uneconomical since the reaction time is then too long; in the extreme case, the reaction here can even stop completely. Low concentrations of an organic substance (that is to say total weight of ether-alcohol and sodium salt of the ether-carboxylic acid) are therefore used in the known processes in order to avoid a high increase in the viscosity. However, this requires subsequent concentration.
It is indeed possible for the dilute aqueous solutions of the alkali metal salts of the ether-carboxylic acids obtained after the oxidation to be concentrated by removing some of the water contained in the solutions by distillation or by acidifying the solutions with strong acids, for example with sulfuric acid, liberating the ether-carboxylic acid and precipitating it, isolating the ether-carboxylic acid and preparing concentrated aqueous solutions after renewed conversion into the alkali metal salts. However, these processes have the following disadvantages:
1. Aqueous solutions of alkali metal salts of ethercarboxylic acids foam greatly during removal of the water by distillation. The profitability of the process is moreover greatly reduced by the energy required for removal of water by distillation. PA0 2. During precipitation of the ether-carboxylic acids with acids, the aqueous phase which remains is polluted by a high salt load, residual ether-carboxylic acids and unreacted ether-alcohol; its disposal is uneconomical. Renewed conversion of the ether-carboxylic acids into their alkali metal salts also leads to an additional increase in the costs of the reaction product.
An additional hindrance occurs in particular if air is used as the oxidizing agent. In this case, the oxidation procedure is made difficult by the foam formed as a result of the surface-active properties of the starting substances and end products The foam issues from the reactor with the waste gas and must be recycled from there back into the reactor after its destruction. The rate of foam formation is always high if air is dispersed in the solution, as is the case, for example, in agitator vessel reactors or bubble column reactors. In agitator vessel reactors in particular, the reaction solution can be converted into a foam-like state by the stirring action, so that mass transfer of the oxygen is prevented and the reaction is inhibited; compare DE-C-2,816,127.
The invention relates to a process for the preparation of alkali metals salts of ether-carboxylic acids of the abovementioned type, in which the above-mentioned disadvantages in respect of the increase in viscosity and the foaming of the reaction mixture are avoided and highly concentrated aqueous solutions of alkali metal salts of the ether-carboxylic acids, for example having a concentration of 20 to 50% by weight, based on the total weight of the solution, can be obtained.
According to the invention, this object is achieved by bringing an aqueous solution, containing an alkali metal hydroxide solution, of the ether-alcohols in a thin layer on a solid support or in the form of fine particles or droplets into contact with oxygen or the gases containing oxygen as the continuous phase, the concentration of the ether-alcohols in the aqueous phase being in the range from at least 0.1, in particular from 0.5 to 15% by weight, based on the total weight of the aqueous phase. Below the stated range the rate of reaction is generally too low so that the concentration should fall below this range only towards the end of the reaction, when the addition of ether-alcohol has ended.
Alkali metal salts of ether-carboxylic acids of the general formula (I) in which the group R can be a straight-chain or branched alkyl group having 1 to 22 carbon atoms can be prepared by the process according to the invention; typical examples of such alkyl groups are methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, dodecyl, tetradecyl, hexadecyl, octadecyl, eicosyl and docosyl. The process according to the invention is particularly suitable for the preparation of alkali metal salts of ether-carboxylic acids in which the radical R is derived from C.sub.12 -C.sub.18 -fatty alcohols, or industrial mixtures thereof, obtainable from animal and/or vegetable fats and oils. The group R can also be an aryl radical, for example a phenyl group, or an aralkyl radical, for example a phenylalkylene group having 1 to 3 carbon atoms in the alkylene radical.
If n&gt;0, the compound of the general formula II is an addition product of ethylene oxide or ethylene oxide and propylene oxide on alcohols of the formula ROH, it being possible, in the case of the ethylene oxide/propylene oxide adducts of the formula II, for the propylene-glycol radicals to be in random or block distribution in the alkoxylate chain, but a terminal ethyleneglycol radical always being present. Addition products of ethylene oxide on alcohols of the formula II are preferred in the context of the invention, so that m=2 is a preferred meaning for the compounds of the formulae I and II.
The increase in viscosity which occurs at higher concentrations of ether-alcohols is avoided if, according to the invention, the reaction is started with only a low ether-alcohol concentration at the beginning and ether-alcohol is metered into the reaction solution continuously or in portions as the reaction progresses further, and in particular at a rate such that the concentration of the ether-alcohols in the reaction mixture does not exceed the value of 15% by weight.
The abovementioned problem of foaming is avoided or considerably reduced by the reaction solution being present in the form of thin layers on a solid support or in the form of fine particles in a continuous phase of oxygen or gases containing oxygen.
According to an advantageous embodiment of the process according to the invention, the oxidation is carried out at a temperature in the range from 40 to 130.degree. C., in particular 60 to 85.degree. C. The rate of reaction is too low below the stated range. Although the reaction can also be carried out above the stated range, this gives only an insignificant increase in the rate of reaction
According to another advantageous embodiment of the process according to the invention, the oxidation is carried out under an oxygen partial pressure of 0.1 to 5 bar. With oxygen-containing gases in particular, foaming is suppressed more and more as the system pressure increases and the effective gas throughput thus decreases. The rate of reaction furthermore increases under certain circumstances as the oxygen partial pressure increases.
According to another advantageous embodiment of the invention, the oxidation is carried out with air. This is another considerable advantage over the processes known from the prior art, in which oxygen is in general used in order to prevent the nitrogen content of air as the oxidizing agent promoting undesirable foaming, and in order to carry out the reaction without a waste gas.
Useful catalysts for use in the process according to the invention are the noble metal catalysts known from the abovementioned prior art, in particular those based on platinum or palladium. Palladium catalysts, for example palladium-un-charcoal, have proved to be particularly suitable for the process according to the invention. The catalyst is preferably introduced into the process in the form of a suspension in the aqueous solution of the ether-alcohols. However, it is also possible for the catalyst to be located on a solid support material, over which the aqueous solution of the ether-alcohols is passed. Possible support materials for this purpose are, for example, active charcoal, graphite, kieselguhr, silica gel, spinels, aluminum oxide or ceramic materials. The catalysts can furthermore also contain combinations of a plurality of noble metals instead of one noble metal, for example mixtures of Pd and Pt, and moreover suitable activators, such as lead, bismuth or cadmium, in the form of their metals or their compounds, including combinations thereof. Suitable catalysts are described in the abovementioned literature and in U.S. Pat. No. 4,607,121.
According to another advantageous embodiment of the process according to the invention, the catalyst is employed in the form of a suspension in a concentration of 0.2 to 3% by weight, based on the total weight of the suspension containing the ether-alcohols and water.
The process according to the invention is in general carried out at pH values of at least 8. Particularly advantageous pH values are at least 9, in particular in the range from 9 to 11. Surprisingly, it has been found that, in contrast to the doctrine of U.S. Pat. No.4,607,121, in spite of these high pH values neither dissolving of the catalyst nor oxidative chain degradation or by-product formation occurs when air is used as the oxidizing agent; the end product of the process according to the invention is Pd-free and the catalyst can be reused after washing with hot water and treatment with hydrogen.
According to another advantageous embodiment of the invention, the oxidation of the ether-alcohols is carried out in a reactor in which the oxygen or the gases containing oxygen and the aqueous phase containing the ether-alcohol, alkali metal hydroxide solution and if appropriate the catalyst are introduced at the top of the reactor, the reaction mixture containing ether-carboxylic acid salts, unreacted ether-alcohol and if appropriate the catalyst is removed at the bottom part of the reactor and the reaction mixture is recycled to the upper part of the reactor for renewed oxidation of as yet unreacted ether-alcohol. It is preferable here for alkali metal hydroxide solution, for maintaining the pH of at least 9, in particular 9 to 11, and ether-alcohol, for maintaining the ether-alcohol concentration of at least 0.1, in particular 0.5 to 15% by weight in the reaction mixture, to be added continuously to the reaction mixture removed at the lower part of the reactor before recycling to the upper part.
According to another advantageous embodiment of the invention, packed columns of the usual construction, such as are described, for example, in Ullmann, Enzyklopadie der Technischen Chemie, 4th edition, volume 3, pages 390 to 392 (1973), Verlag Chemie, Weinheim, as parallel flap packed columns, are used for carrying out the process.
The packings to be employed in the packed columns advantageously have a high intermediate volume so that the gas speed and the rate of foaming does not become too high. Typical examples of suitable packings are known from Ullmanns Enzyklopadie der technischen Chemie, 4th edition, volume 2, page 529 (1972) and 5th edition, volume B3, pages 4-82 to 4-83 (1988); the use of Pall rings, Novalox saddles, Berl saddles, Intralox saddles and Interpack bodies is particularly preferred. Ordered packings such as are described in volume 2 of the 4th edition of the abovementioned encyclopedia, pages 533-534, for example of the Sulzer packing type, can furthermore also be employed. Finally, it is also possible to employ bulk catalysts or catalyst fixed beds instead of bulk packing. Ordered catalyst packings, for example in honeycomb form, can also be employed.
According to another advantageous embodiment of the invention, the reaction mixture removed at the lower end of the column is recycled, after the pH and the ether-alcohol concentration has been adjusted, to the upper part of the column for renewed reaction until a concentration of the ether-carboxylic acid salt of 20 to 50% by weight, based on the total weight of the solution, is reached and the ether-alcohol metered in has reacted.
The invention is illustrated in more detail below with the aid of the drawing and a preferred exemplary embodiment.